I think it depends on both the extension and the registrar (for some odd reason). For example, us and org almost always resolve for me within minutes or even in seconds (not hrs) at all the firms I use.
However, sometimes com will also resolve in a few minutes at one firm but at another registrar they can take hrs or even a full day or 2 at times. For some odd reason the slowness with com happens a lot at Enom, especially on weekends, where perhaps staff looks at it and not there, I am guessing.